Medically important arthropods continue to pose a substantial menace to the health of humans; malaria transmission accounts yearly for 1.5 x 10 to the 6th power deaths worldwide, and the transmission of viruses and filarioid nematodes, as well as the insatiable bloodsucking causes additional mortality and suffering. Currently, the major means of control is by chemical treatments, but because of the increase in pesticide resistance and the necessity to reduce chemical contamination of the environment, there is a pressing need to develop biological control methods. Among the numerous organisms available for biological control of medically-important arthropods, the mermithid nematodes have outstanding potential. Although a few mermithid species can be propagated readily in their host species, there are many that are not available for biocontrol studies because of the difficulties in host propagation, e.g., blackflies. One alternative is to culture the mermithid nematodes in vitro. However for successful in vitro culture of mermithid nematodes, one rerequisite is a knowledge of the nutritional and physiological requirements of the parasitic stage. This proposal is an attempt to reach this goal by using Romanomermis culicivorax, a mermithid parasite of mosquitoes, as a model organism. By biochemical analysis of major chemical factors in the host hemolymph and their changes during parasitism it is hoped that a rational nutritional medium can be designed in the future. Specifically, amino acid, lipid, organic acids, free bases and their derivatives, pH, osmotic pressure, ions, and hemin composition of the hemolymph will be determined. In addition, the possibility of parasite uptake of host proteins and other important metabolites will be determined.